Processing facilities, such as manufacturing plants, chemical plants and oil refineries, are typically managed using process control systems. Valves, pumps, motors, heating/cooling devices, and other industrial equipment typically perform actions needed to process materials in the processing facilities. Among other functions, the process control systems often control industrial operation in the processing facilities.
A very common problem encountered in many industries, such as the oil and petrochemical industry, is compensating for variations in the flow rate of fluids comprising liquid coming into a particular processing unit. Such disturbances are usually common and ordinary events in the routine operation of the process.
One strategy has been to include one or more surge tanks in the liquid flow lines or to utilize certain volume capacity ranges within existing vessels to provide temporary capacity for smoothing out the flow variations. The liquid levels in these vessels, e.g., surge tanks, bottoms of fractionation columns and accumulators, and so forth, may then be allowed to vary within limits so that the outlet flow changes from these vessels are significantly smaller than the instantaneous inlet flow changes. Each liquid level thus acts as a buffer for the downstream units. Thus, this surge capacity, which may be receiving flow from a number of different units, by allowing the level in the surge tank to deviate from its setpoint while staying within allowable limits, attenuates the effects of any feed flow disturbance so that the disturbances do not propagate quite as strongly and the operation of the process is steadier.
A useful surge volume control algorithm should have several important characteristics. The level in the surge vessel should not exceed the high and low level limits to ensure that the vessel will not overflow or empty. In the absence of any disturbance over a long period of time, the level should line out at the target level (setpoint). The available surge volume should be utilized effectively to minimize the effect of a feed rate change and other process disturbances on the downstream process. The algorithm should be able to handle surge vessels of all shapes, such as vertical cylindrical, horizontal cylindrical, spherical, and vessels with internal baffles and various end types. The method should be relatively simple so that it can be easily maintained and executed at high speed on a control system platform. Also, tuning the controller should not be difficult and should not require much effort.
Microprocessor based Proportional, Integral and Derivative (PID) controllers are currently the most commonly used controllers for level control to reduce variations in the flow supplied to a downstream process. However, the PID algorithms run by PID controllers are generally known to have two significant limitations. First, PID algorithms cannot generally address non-linearities. Second, PID algorithms cannot be used to specify high and low limits for liquid levels explicitly. Moreover, if the inlet flow has a large noise component, such as due to an upstream process that is noisy, control using a PID algorithm becomes increasingly ineffective. What is needed is a non-linear level controller (NLLC) and related algorithm to more effectively allow surge vessels to absorb incoming fluctuations in the inlet flow so that the outlet liquid flow to a downstream process is more consistent.